The present invention relates generally to burnalbe absorbers (also called burnable poisons) for nuclear reactors and, more particularly, to an improved burnable absorber coating for nuclear fuel.
It is known that nuclear fuel may have various shapes such as plates, columns, and even fuel pellets disposed in end-to-end abutment within a tube or cladding made of a zirconium alloy or stainless steel. The fuel pellets contain fissionable material, such as uranium dioxide, thorium dioxide, plutonium dioxide, or mixtures thereof. The fuel rods are usually grouped together to form a fuel assembly. The fuel assemblies are arranged together to constitute the core of a nuclear reactor.
It is well known that the process of nuclear fission involves the disintegration of the fissionable nuclear fuel material into two or more fission products of lower mass number. Among other things the process also includes a net increase in the number of available free neutrons which are the basis for a self-sustaining reaction. When a reactor has operated over a period of time the fuel assembly with fissionable material must ultimately be replaced due to depletion. Inasmuch as the process of replacement is time consuming and costly, it is desirable to extend the life of a given fuel assembly as long as practically feasible. For that reason, deliberate additions to the reactor fuel of parasitic neutron-capturing elements in calculated small amounts may lead to highly beneficial effects on a thermal reactor. Such neutron-capturing elements are usually designated as "burnable absorbers" if they have a high probability (or cross section) for absorbing neutrons while producing no new or additional neutrons or changing into new absorbers as a result of neutron absorption. During reactor operation the burnable absorbers are progressively reduced in amount so that there is a compensation made with respect to the concomitant reduction in the fissionable material.
The life of a fuel assembly may be extended by combining an initially larger amount of fissionable material as well as a calculated amount of burnable absorber. During the early stages of operation of such a fuel assmebly, escessive neutrons are absorbed by the burnable absorber which undergoes transformation to elements of low neutron cross section which do not substantially affect the reactivity of the fuel assembly in the latter period of its life when the availability of fissionable material is lower. The burnable absorber compensates for the larger amount of fissionable material during the early life of the fuel assembly, but progressively less absorber captures neutrons during the latter life of the fuel assembly, so that a long life at relatively constant fission level is assured for the fuel assembly. Accordingly, with a fuel assembly containing both fuel and burnable absorber in carefully proportioned quantity, an extended fuel assembly life can be achieved with relatively constant neutron production and reactivity.
Burnable absorvers which may be used include boron, gadolinium, samarium, europium, and the like, which upon the absorption of neutrons result in isotopes of sufficiently low neutron capture cross section so as to be substantially transparent to neutrons.
The incorporation of burnable absorbers in fuel assemblies has been recognized in the nuclear field as an effective means of increasing fuel capacity and thereby extending core life. Burnable absorbers are used either uniformly mixed with the fuel (i.e., distributed absorber) or are placed discretely as separate elements in the reactor, so arranged that they burn out or are depleted at about the same rate as the fuel. Thus, the net reactivity of the core is maintained relatively constant over the active life of the core.
In U.S. Pat. No. 3,108,936 a magnesium zirconate or zirconium carbide protective fluid-tight coating is applied on uranium carbide fuel pellets allegedly so that if the fuel rod leaked, moisture from the water coolant would not reach the uranium carbide to react with it and change it to an unusable powdery oxide.
U.S. Pat. No. 3,427,222 discloses a uranium dioxide fuel pellet substrate coated with a mixture of uranium dioxide and a zirconium diboride burnable poison applied by a plasma spraying technique (see column 4, "Example I"). That patent also disclosed a uranium dioxide fuel pellet substrate coated with the burnable poison boron applied by chemical vapor deposition, and the patent noted that the deposition rate was slow at low temperatures while the coating was not as adherent at high temperatures (see column 5, "Example III").
It is known that a nuclear fuel contained in an aluminum can may be coated with a layer of niobium to prevent the fuel from reacting with the can (British Pat. No. 859,206; page 1; lines 12-30). It is also known that minute nuclear fuel particles, such as uranium dioxide particles, may be coated with a single layer or several layers of the same or different non-absorber materials, including niobium, for such purposes as protecting the fuel from corrosion and helping to retain the products of fission. The coatings may be applied by various techniques, such as depositing from a vapor of the coating material, depositing from a decomposing vapor, and electroplating (British Pat. No. 933,500).
Japanese Pat. No. 52-3999 discloses a nuclear fuel first coated with a thin layer of a material (such as niobium) to absorb fission fragments and then coated with a main coating material (such as a Zircaloy). The patent apparently does not concern burnable absorber coatings, and is not relevant to the present invention.
In Dispersion Fuel Elements, an AEC Monograph by A. N. Holden published in 1967 by Gordon and Breach of New York there is mentioned coating fuel particles in dispersion fuels to prevent interaction of the particles with the matrix and to retain fission products (page 30). Uranium dioxide coated with niobium by vapor-phrase reduction is disclosed (page 48). Also disclosed is uranium dioxide coated with chromium, by vapor-phase reduction using chromium dichloride, which was deposited over a niobium undercoat (page 48).
The present inventors are aware of the earlier documented work disclosed in a commonly assigned U.S. patent application entitled "Coating a Uranium Dioxide Nuclear Fuel With a Zirconium Diboride Burnable Poison", by Walston Chubb, concomitantly filed with the present application, wherein spalling problems with chemically vapor depositing zirconium diboride on uranium dioxide were overcome by first deposition (by sputtering, chemical vapor deposition, etc.) a thin undercoat layer of niobium (of between about 3 microns and about 6 microns in thickness) on the uranium dioxide and then chemically vapor depositing the zirconium diboride on he niobium layer.
Fuel pellets coated with a boron containing burnable absorber such as elemental boron, boron-10 isotope (the isotope of elemental boron having the burnable absorber property), zirconium diboride, boron carbide, boron nitride, and the like suffer from varying degrees of moisture adsorption. For example, uranium dioxide fuel pellets coated with zirconium diboride, after manufacture, must be furnace dried in a time consuming operation and then loaded into the fuel rods in a low humidity glove box environment. This is required because the zirconium diboride, being hygroscopic, takes on a thin layer of moisture (moisture adsorption) from the air itself. The added lengthy drying step (typically about 1 to 3 hours at temperatures of 200.degree.-600.degree. C. in a vacuum of less than or equal to 1 torr) and humidity controlled pellet loading environment add to the time, complexity and the cost of the nuclear fuel processing line. Moisture is to be avoided in nuclear fuel because, as is known to those skilled in the art, excessive hydrogen in the fuel pellet, appearing mostly as moisture, causes hydriding of the Zircaloy fuel rod when the hydrogen is released from the fuel pellet during operation of the reactor. The resulting corrosive effects of the hydriding may cause a breach in the fuel rod material and result in radioactive particles leaking into the water circulating through the reactor.